1. Field of the Invention
This invention relates in general to an active vibration damping device which is mounted on a desired object whose vibration is to be damped, for actively damping vibration of this object. More particularly, the present invention is concerned with a pneumatically operated active vibration damping device which includes a mass member which is oscillated by a periodic change of an air pressure applied thereto, for applying an oscillating force to the object so as to provide an active vibration damping effect.
2. Discussion of the Related Art
A dynamic damper is widely known, as a vibration damping device for damping vibration of a desired object such as the body of an automotive vehicle, whose vibration is to be damped. In recent years, there has been proposed an active vibration damping device including a mass member which is oscillated to apply an oscillating force to the object, for thereby exhibiting an improved active vibration damping effect. An example of this active vibration damping device is disclosed in JP-A-3-292219 and JP-A-6-235438, wherein a vibration system is constituted by a mounting member for attachment to the object, a spring member, and a mass member connected to the mounting member through the spring member. This active vibration damping device includes an electromagnetic actuator mechanism for oscillating the mass member of the vibration system, so as to provide an increased active damping effect with respect to the object, based on the oscillation of the mass member.
There is also proposed an active vibration damping device of another type wherein the mass member is exposed to an air chamber formed between the mounting member and the mass member, so that the mass member is oscillated by a periodic change of the air pressure in the air chamber, at a frequency corresponding to the frequency at which a switch valve is alternately placed in two operating positions, namely, a "vacuum position" for communication of the air chamber with a vacuum source, and an "atmospheric position" for communication of the air chamber with the atmosphere. This pneumatically operated type of active vibration damping device does not require a heavy member such as an electromagnetic actuator to be incorporated therein, resulting in reduction in the number of the required components, the size and weight and the required amount of electric power consumption of the device.
In the pneumatically operated type of active vibration damping device, it is important and desirable to control the switch valve and the negative pressure in the air chamber so that not only the frequency and phase but also the amplitude of the oscillation of the mass member correspond to those of the vibration of the object to be damped, in order to improve the active damping effect with respect to the vibration of the object.
To meet the need indicated above, the inventors of the present invention have considered it possible to (a) detect the frequency, phase and amplitude of the vibration of the object, by using a suitable sensor such as an accelerometer or acceleration sensor, or estimate those frequency, phase and amplitude according to predetermined data maps, (b) control the frequency and phase of the switching operation of the switch valve, so that the frequency and phase of the oscillation of the mass member correspond to those of the vibration of the object, and (c) control the negative pressure in the air chamber so that the amplitude of the oscillation of the mass member corresponds to that of the vibration of the object.
Where the pneumatically operated active vibration damping device is used for an automotive vehicle, and the air intake system of the engine of the vehicle is used as the vacuum source, it is difficult to regulate the negative pressure in the air intake system (vacuum source), and is therefore difficult to obtain the oscillation of the mass member whose amplitude corresponds to that of the vibration of the object to be damped. The active vibration damping device does not provide a sufficiently high active vibration damping effect, or may even deteriorate the vibrating condition of the object, if the amplitude of the oscillation of the mass member does not accurately correspond to that of the vibration of the object.
In the light of the above, the inventors have also considered it possible to control the level of the negative pressure in the air chamber depending upon the amplitude of the vibration of the object, by controlling the duty ratio of the switch valve based on the amplitude of the vibration of the object. The duty ratio of the switch valve is interpreted to mean a ratio of a time in which the switch valve is held in its vacuum or atmospheric position, to the entire period of the switching operation of the switch valve. Where the amplitude of the oscillation of the object is considerably small or large, the duty ratio of the switch valve is considerably smaller or larger than the intermediate value of 0.5, in order to obtain the oscillation amplitude of the mass member which corresponds to the vibration amplitude of the object. However, an extensive analysis by the present inventors regarding this duty-ratio control of the switch valve has, revealed that a duty ratio value of the switch valve which is considerably smaller or larger than the intermediate value of 0.5 would cause a considerably large amount of distortion or deviation of the waveform of the oscillation of the mass member (waveform of the active damping force acting on the object), with respect to the waveform of the vibration of the object to be damped, causing generation of higher harmonics components of the primary frequency of the oscillation (higher harmonics components of the frequency of the vibration to be damped).